1. Technical Field
The present invention relates to a method of manufacturing a test circuit on a silicon wafer.
The present invention relates more particularly to the manufacture of a calibration circuit enabling the calibration of a RF integrated circuit probe.
The present invention also relates to the electrical testing of RF integrated circuits.
2. Description of the Related Art
RF integrated circuits include inputs/outputs provided for receiving or emitting electrical signals with a frequency on the order of some hundreds of megaHertz to some gigaHertz and belonging to the range of radio frequencies. Examples include, in particular, integrated circuits comprising analog functions of modulation and demodulation, mixers, amplifiers, filters, voltage controlled oscillators (VCOs), phase locked loops (PLLs), etc., for use in radio receivers, television receivers, mobile radiotelephones, GPS receivers, and the like. Also, some microprocessor digital integrated circuits today reach operating frequencies of about 500-800 MHz, which are included in the range of radio frequencies.
Generally, RF or non-RF integrated circuits are collectively manufactured on a silicon plate called a wafer and are electrically tested by means of a testing probe before cutting the wafer into separate components. This testing step "on wafer" enables location and elimination of faulty integrated circuits before their mounting in a housing or on an interconnection support.
The electrical test of RF integrated circuits requires taking a particular care because, in this frequency range, electrical signals have short wavelengths and various reflection and phase rotation phenomena are seen in the test probes, corrupting the electrical measures by creating interference and changing electrical levels.
Thus, specialized firms have developed RF test probes presenting satisfactory high frequency characteristics. In particular, the firm Cascade.TM. Microtech, Oregon 96505, USA, provides RF probe tips ("transmission line probes") with the reference "Air Coplanar" and membrane probes ("RFIC membrane probe cards") provided with microstrip HF conductors and nickel contact bumps. Such probes present a bandwidth of several tenths of GigaHertz, a low reflection coefficient S11 and a transmission coefficient S21 with an attenuation lower than 3 dB.
Furthermore, manufacturers of measurement devices such as the firm Teradyne.TM. have developed test stations (series A580) comprising RF measurement ports provided with a network analyzer ("vector network analyzer") able to determine the "S" parameters (S11, S12/S21, and S22) of a test probe by means of the OSL ("Open, Short, Load") method. As is well known to those skilled in the art, the OSL method consists in performing three measures by successively applying at least three standard loads at the output of a test probe, i.e. a load with an infinite impedance, a load with a zero impedance, and a load with a non-zero impedance, generally of 50.OMEGA.. From these three measures, the network analyzer determines the "S" parameters of the probe. Once the "S" parameters are known, the test station performs an automatic error correction during later measures, compensating for the influence of these parameters and obtaining accurate and reliable measures.
At the present time, the standard loads used for carrying out the OSL method are thin layer circuits on ceramic plates, calibrated by a national laboratory of metrology. There can be found therefore on the market calibration circuits comprising various elementary standard structures, like open circuits, short-circuits, and ohmic loads of 50.OMEGA..
Such calibration circuits have several drawbacks, and in particular a high cost and a low lifetime. At each calibration, the tips or contact bumps of the probes to be calibrated are applied with strength on the contact pads of the calibration circuits, so as to break a superficial oxide layer which is formed at the contact of air and provide a good electrical contact ("cold welding").
Furthermore, thin layer calibration circuits do not present the same thickness as silicon wafers and require an adjustment of the test plate for their installation, necessarily followed by another adjustment when installing the wafer. This drawback is added to the fact that RF probes require several calibrations during the test of a set of integrated circuits. These various calibrations involve the intervention of a specialized engineer and represent 5% to 10% of the time dedicated to the electrical test.
The present invention thus recognizes that thin layer calibration circuits do not allow the implementation of a "on line" test method, that is on the production place itself, which gives satisfactory test results and is adapted to mass production which can represent several millions of RF integrated circuits.
Thus, an object of the present invention is to provide a calibration circuit with a low manufacturing cost which allows the calibration of a test probe in a simple manner.
Another object of the present invention is to provide a method allowing the fabrication of a calibration circuit together with integrated circuits to be marketed on a silicon wafer.